diff --git a/contracts/.gas-snapshot b/contracts/.gas-snapshot
index 73b8bfc..2e5a106 100644
--- a/contracts/.gas-snapshot
+++ b/contracts/.gas-snapshot
@@ -1 +1,5 @@
-FooTest:test_Example() (gas: 8662)
\ No newline at end of file
+ScKeystoreTest:test__addUser__addsUser__whenUserInfoIsValid() (gas: 56777)
+ScKeystoreTest:test__addUser__reverts__whenUserAlreadyExists() (gas: 61532)
+ScKeystoreTest:test__addUser__reverts__whenUserInfoIsMalformed() (gas: 9384)
+ScKeystoreTest:test__getUser__returnsUserInfo__whenUserExists() (gas: 60956)
+ScKeystoreTest:test__userExists__returnsFalse__whenUserDoesNotExist() (gas: 7976)
\ No newline at end of file
diff --git a/contracts/script/Deploy.s.sol b/contracts/script/Deploy.s.sol
index 68f0689..8d6b318 100644
--- a/contracts/script/Deploy.s.sol
+++ b/contracts/script/Deploy.s.sol
@@ -1,13 +1,13 @@
 // SPDX-License-Identifier: UNLICENSED
 pragma solidity >=0.8.19 <=0.9.0;
 
-import { Foo } from "../src/Foo.sol";
+import { ScKeystore } from "../src/ScKeystore.sol";
 import { BaseScript } from "./Base.s.sol";
 import { DeploymentConfig } from "./DeploymentConfig.s.sol";
 
 contract Deploy is BaseScript {
-    function run() public returns (Foo foo, DeploymentConfig deploymentConfig) {
+    function run() public returns (ScKeystore scKeystore, DeploymentConfig deploymentConfig) {
         deploymentConfig = new DeploymentConfig(broadcaster);
-        foo = new Foo();
+        scKeystore = new ScKeystore();
     }
 }
diff --git a/contracts/src/Foo.sol b/contracts/src/Foo.sol
deleted file mode 100644
index d69be05..0000000
--- a/contracts/src/Foo.sol
+++ /dev/null
@@ -1,8 +0,0 @@
-// SPDX-License-Identifier: UNLICENSED
-pragma solidity >=0.8.19;
-
-contract Foo {
-    function id(uint256 value) external pure returns (uint256) {
-        return value;
-    }
-}
diff --git a/contracts/src/IScKeystore.sol b/contracts/src/IScKeystore.sol
index 237e806..ccc1449 100644
--- a/contracts/src/IScKeystore.sol
+++ b/contracts/src/IScKeystore.sol
@@ -7,13 +7,13 @@ struct KeyPackage {
 }
 
 struct UserInfo {
-    KeyPackage[] keyPackages;
+    uint256[] keyPackageIndices;
     bytes signaturePubKey;
 }
 
 interface IScKeystore {
     function userExists(address user) external view returns (bool);
-    function addUser(UserInfo calldata userInfo) external;
+    function addUser(bytes calldata signaturePubKey, KeyPackage calldata keyPackage) external;
     function getUser(address user) external view returns (UserInfo memory);
     function addKeyPackage(KeyPackage calldata) external;
     function getAvailableKeyPackage(address user) external view returns (KeyPackage memory);
diff --git a/contracts/src/ScKeystore.sol b/contracts/src/ScKeystore.sol
new file mode 100644
index 0000000..bea1119
--- /dev/null
+++ b/contracts/src/ScKeystore.sol
@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity 0.8.24;
+
+import { IScKeystore, UserInfo, KeyPackage } from "./IScKeystore.sol";
+
+error UserAlreadyExists();
+error MalformedKeyPackage();
+error MalformedUserInfo();
+error UserDoesNotExist();
+
+contract ScKeystore is IScKeystore {
+    event UserAdded(address user, bytes signaturePubKey);
+    event UserKeyPackageAdded(address indexed user, uint256 index);
+
+    mapping(address user => UserInfo userInfo) private users;
+    KeyPackage[] private keyPackages;
+
+    function userExists(address user) public view returns (bool) {
+        return users[user].signaturePubKey.length > 0;
+    }
+
+    function addUser(bytes calldata signaturePubKey, KeyPackage calldata keyPackage) external {
+        if (signaturePubKey.length == 0) revert MalformedUserInfo();
+        if (keyPackage.data.length == 0) revert MalformedKeyPackage();
+        if (userExists(msg.sender)) revert UserAlreadyExists();
+
+        keyPackages.push(keyPackage);
+        uint256 keyPackageIndex = keyPackages.length - 1;
+
+        users[msg.sender] = UserInfo(new uint256[](0), signaturePubKey);
+        users[msg.sender].signaturePubKey = signaturePubKey;
+        users[msg.sender].keyPackageIndices.push(keyPackageIndex);
+
+        emit UserAdded(msg.sender, signaturePubKey);
+    }
+
+    function getUser(address user) external view returns (UserInfo memory) {
+        return users[user];
+    }
+
+    function addKeyPackage(KeyPackage calldata keyPackage) external {
+        if (keyPackage.data.length == 0) revert MalformedKeyPackage();
+        if (!userExists(msg.sender)) revert UserDoesNotExist();
+
+        keyPackages.push(keyPackage);
+        uint256 keyPackageIndex = keyPackages.length - 1;
+        users[msg.sender].keyPackageIndices.push(keyPackageIndex);
+
+        emit UserKeyPackageAdded(msg.sender, keyPackageIndex);
+    }
+
+    function getAvailableKeyPackage(address user) external view returns (KeyPackage memory) {
+        UserInfo memory userInfo = users[user];
+        uint256 keyPackageIndex = userInfo.keyPackageIndices[userInfo.keyPackageIndices.length - 1];
+        return keyPackages[keyPackageIndex];
+    }
+
+    function getAllKeyPackagesForUser(address user) external view returns (KeyPackage[] memory) {
+        UserInfo memory userInfo = users[user];
+        KeyPackage[] memory userKeyPackages = new KeyPackage[](userInfo.keyPackageIndices.length);
+        for (uint256 i = 0; i < userInfo.keyPackageIndices.length; i++) {
+            userKeyPackages[i] = keyPackages[userInfo.keyPackageIndices[i]];
+        }
+        return userKeyPackages;
+    }
+}
diff --git a/contracts/test/Foo.t.sol b/contracts/test/Foo.t.sol
deleted file mode 100644
index 6b15158..0000000
--- a/contracts/test/Foo.t.sol
+++ /dev/null
@@ -1,26 +0,0 @@
-// SPDX-License-Identifier: UNLICENSED
-pragma solidity >=0.8.19 <0.9.0;
-
-import { Test, console } from "forge-std/Test.sol";
-
-import { Deploy } from "../script/Deploy.s.sol";
-import { DeploymentConfig } from "../script/DeploymentConfig.s.sol";
-import { Foo } from "../src/Foo.sol";
-
-contract FooTest is Test {
-    Foo internal foo;
-    DeploymentConfig internal deploymentConfig;
-
-    address internal deployer;
-
-    function setUp() public virtual {
-        Deploy deployment = new Deploy();
-        (foo, deploymentConfig) = deployment.run();
-    }
-
-    function test_Example() external {
-        console.log("Hello World");
-        uint256 x = 42;
-        assertEq(foo.id(x), x, "value mismatch");
-    }
-}
diff --git a/contracts/test/ScKeystore.t.sol b/contracts/test/ScKeystore.t.sol
new file mode 100644
index 0000000..b8a44b6
--- /dev/null
+++ b/contracts/test/ScKeystore.t.sol
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.19 <0.9.0;
+
+import { Test } from "forge-std/Test.sol";
+import { Deploy } from "../script/Deploy.s.sol";
+import { DeploymentConfig } from "../script/DeploymentConfig.s.sol";
+import "../src/ScKeystore.sol"; // solhint-disable-line
+
+contract ScKeystoreTest is Test {
+    ScKeystore internal s;
+    DeploymentConfig internal deploymentConfig;
+    address internal deployer;
+
+    function setUp() public virtual {
+        Deploy deployment = new Deploy();
+        (s, deploymentConfig) = deployment.run();
+    }
+
+    function addUser() internal {
+        KeyPackage memory keyPackage = KeyPackage({ data: new bytes[](1) });
+        s.addUser("0x", keyPackage);
+    }
+
+    function test__userExists__returnsFalse__whenUserDoesNotExist() public view {
+        assert(!s.userExists(address(this)));
+    }
+
+    function test__addUser__reverts__whenUserInfoIsMalformed() public {
+        vm.expectRevert(MalformedUserInfo.selector);
+        s.addUser("", KeyPackage({ data: new bytes[](0) }));
+    }
+
+    function test__addUser__reverts__whenUserAlreadyExists() public {
+        addUser();
+        vm.expectRevert(UserAlreadyExists.selector);
+        addUser();
+    }
+
+    function test__addUser__addsUser__whenUserInfoIsValid() public {
+        addUser();
+        assert(s.userExists(address(this)));
+    }
+
+    function test__getUser__returnsUserInfo__whenUserExists() public {
+        addUser();
+        UserInfo memory userInfo = s.getUser(address(this));
+        assert(userInfo.signaturePubKey.length == 2);
+        assert(userInfo.keyPackageIndices.length == 1);
+    }
+
+    function test__getAllKeyPackagesForUser__returnsKeyPackages__whenUserExists() public {
+        addUser();
+        KeyPackage[] memory keyPackages = s.getAllKeyPackagesForUser(address(this));
+        assert(keyPackages.length == 1);
+    }
+}